Liquid Crystal Displays (LCDs) are widely used in a broad range of display devices and compete favorably with the more conventional cathode-ray tube (CRT) monitor for numerous display applications. While direct-view LCDs continually improve in resolution, speed, and overall performance, however, display brightness can still be disappointing when compared against CRTs. This shortcoming is particularly noticeable over larger viewing angles.
An inherent problem with LCD displays that limits brightness relates to polarization dependence. In typical applications, the LCD device itself has a pair of absorptive polarizers that absorb half of the unpolarized light emitted from the light source. Thus, even where brighter light sources can be provided, a considerable portion of this light is still discarded.
One solution to this problem has been to use a reflective polarizer, such as Vikuiti™ Dual Brightness Enhancement Film, manufactured by 3M, St. Paul, Minn. or wire grid polarizer wire-grid polarizer available from Moxtek, Inc., Orem, Utah. These devices transmit only the light having the desired polarization for the LCD and reflect light of the orthogonal polarization, which can be re-oriented by illumination components so that it is eventually used.
Reflective polarizers work well for a portion of the light, particularly for light at incident angles near normal with respect to the reflective polarizer. However, light incident at angles diverging from normal, or large-angle light, is not used efficiently. This inefficiency can be difficult to remedy, since some deliberate scattering of light is typically performed within the light guide plate (LGP). Scattering elements, such as printed dots or an etched pattern, are often necessary for uniformizing the light in a conventional backlight system. Thus, uniformization and polarization elements may tend to work at cross-purposes, requiring some compromise to achieve both suitable brightness and acceptable uniformity.
One approach using a reflective polarizer, or polarizing beamsplitter, positions this polarizing component at the bottom face of a light guide plate without any scattering element, as disclosed in U.S. Pat. No. 6,285,423 to Li et al. (see '423 Li et al. FIG. 1a, numeral 20). With this type of arrangement, light transmitted through the reflective polarizer is redirected as illumination output at near normal angles. Light reflected by the reflective polarizer is directed to a polarization converter for changing the polarization of at least a portion of this light and redirecting it for eventual output.
An alternate approach using a reflective polarizer positions this element at the top of a light guide plate without any scattering element, as disclosed in U.S. Pat. No. 6,443,585 to Saccomanno (see '585 Saccomanno FIG. 1 numeral 8). This approach generally provides higher light extraction efficiency due to reduced loss in scattering, but it does not provide a satisfactory polarizing effect.
In order to understand the problems with reflective polarizer use encountered with both the Li et al. '423 and Saccomanno '585 approaches, it is useful to observe how polarized light is handled within the light guide plate of an illumination system. To do this, compare the schematic view of FIG. 1A in the present disclosure with FIG. 1A of the Li et al. U.S. Pat. No. 6,285,423 disclosure and FIG. 1 of the Saccomanno U.S. Pat. No. 6,443,585 disclosure. A beam of light 10 is emitted from a light source 14 that is positioned at the entrant plane of a light guide plate 12. The incident angle at the entrant plane is θi, which is between 0 and 90° in the most general case. Beam of light 10 is then coupled into light guide plate 12 and is incident on a reflective polarizer 20. Polarization states are represented using standard schematic notation: S-polarization is represented by a large dot along the line of light, P-polarization is shown by a line orthogonal to the line of light. Three light extraction structures 25 for defeating TIR at the surface of light guide plate 12 are shown; a number of different films or structures, well known in the backlighting illumination arts, could be utilized for this purpose. A polarization converter, such as a quarter-wave film or plate, is located on bottom face 18 or on end face 19, or on both bottom and end faces 18 and 19.
A detailed description of a conventional reflective polarizer 20 can be found in the Li et al. U.S. Pat. No. 6,285,423 disclosure. Briefly, according to conventional practice, reflective polarizer 20 can have a number of possible forms, including: (1) a stack of 1.38/2.35 dielectric layers deposited on polycarbonate substrates; (2) a stack of metal/dielectric layers on a substrate; (3) a layer of birefringent material such as liquid crystalline material sandwiched between two substrates; or (4) a stretched plastic film with a blend of birefringent material and isotropic material, as shown in FIGS. 8c, 10c, and 12c of U.S. Pat. No. 6,285,423. However, it must be emphasized that the polarizing effect is achieved only when the light is within the limited acceptance angle range of the device. According to configurations shown in the Li et al. '423 disclosure, the acceptance angle (referred to as the incident angle in the Li et al. '423 patent) is either in the range 69° to 79° (see Li et al. '423 patent, FIG. 9); 62° to 82° (see Li et al. '423 patent, FIG. 11); or 70° to 84° (see Li et al. '423 patent, FIG. 13).
When the following condition is met for angle θTIR, the reflective polarizer transmits one polarization and reflects the other polarization due to total internal reflection (TIR), thus separating two polarization states for all light trapped in the light guide plate:
                                          θ            TIR                    =                                                    sin                                  -                  1                                            ⁡                              (                                                      n                    o                                                        n                    LGP                                                  )                                      ≤                                          90                0                            -                                                sin                                      -                    1                                                  ⁡                                  (                                      1                                          n                      LGP                                                        )                                                                    ⁢                                  ⁢        and        ⁢                                  ⁢                                            n              e                        =                          n              LGP                                ,                                    (                  equation          ⁢                                          ⁢          1                )            where nLGP is the index of refraction of the light guide plate substrate, no is the extraordinary index of refraction, ne is the ordinary index of refraction.
For a better understanding of the limitations on light incident angle inherent to the conventional approach used in the Li et al. '423 disclosure, it is particularly instructive to take a closer look at the case when reflective polarizer 20 is a layer of birefringent material with extraordinary index ne and ordinary index no. In this particular case, the direction of ne is parallel to the light source 14 or perpendicular to the plane of incidence shown in FIG. 1A. Using the values used in Li et al. '423 by way of example:    ne=nLGP=1.589 and    no=1.5
The light trapped in the light guide has an acceptance angle θa, which is bounded as follows:
                                          90            0                    -                                    sin                              -                1                                      ⁡                          (                              1                                  n                  LGP                                            )                                      ≤                  θ          a                <                  90          0                                    (                  equation          ⁢                                          ⁢          2                )            that is, for a substrate with index nLGP=1.589:51°≦θa<90°.Thus, light between 51 and 90 degrees is within the acceptance angle for the light guide.
However, a good polarization separation is provided only where there is total internal reflection, that is, only for light with incident angle greater than
      θ    TIR    =                    sin                  -          1                    ⁡              (                              n            o                                n            LGP                          )              .  With a conventional light guide plate, this lower threshold is at 71° for no=1.5 and nLGP=1.589. This means that, according to the teaching disclosed in the Li et al. '423 patent, reflective polarizer 20 does not provide satisfactory polarization separation effect for light with incident angles between 51° and 71°. Only for light that is in the 71-90 degree range is acceptable polarization separation provided.
Table 1 and the comparative examples of accompanying FIGS. 1B, 1C, and 1D show the shortcomings of the conventional approach to reflective polarizer structure, such as the approach described in the Li et al. '423 disclosure (see Li et al. '423 col. 8, lines 66-67). These examples show a polycarbonate substrate, having nLGP=1.589. In FIGS. 1B-1D, the transmission value for light of S-polarization (polarized perpendicular to the plane of incidence and designated by the circles in FIG. 1A) at various acceptance angles θa is given by curve T90 (filled squares). The reflectivity for the orthogonal polarization, or P-polarization (polarized parallel to the plane of incidence and designated by the lines in FIG. 1A) is given by curve R0 (open triangles). As shown by dashed box Q in FIG. 1B, good separation of light in the range from 51-90 degrees is desired for polarization of light from the light guide plate. Light separation is considered to be acceptable when both T90 and R0 values exceed about 0.8, indicating that 4:1 or better polarization separation is achieved.
In Table 1, exemplary values are given for indices nLGP, ne, and no. The depth D is the thickness of the birefringent polarization material. Of particular interest for overall performance is the overlap angle range and effective acceptance angle θa range given in the right-most columns.
It is to be noted that the 89 degree value shown in tables and used in description in the present disclosure is used to express an angular value for acceptance angle θa that can approach 90 degrees as a limit, but is less than 90 degrees.
For the example of FIG. 1B with depth D=5 μm, the overlapping angle θ is between 71° and 89°, which is about half the desired 51-90 degree range. Light between 51° and 71° is not well-polarized. The example of FIG. 1C exhibits at least some improvement by using a birefringent layer that has very low ordinary index no=1.389. However, this is a theoretical material that yields a very large birefringence of 0.2, given ne=1.589. It would be unusual to find a usable material having a birefringence of this value for sheet reflective polarizer use, given ne=1.589. Even if such a material were available, however, the overlapping angle θa is only between 61° and 89°, which still fails to span the desired range.
The example of FIG. 1D comes just a bit closer to the desired performance by using the theoretical material of the FIG. 1C example and varying the depth D of the birefringent layer D=0.5 μm.
TABLE 1Summary of Parameters and Performance for ConventionalPolarizers (LGP substrate nLGP = 1.589)θa ofθRθToverlapD(where(where(T90 > 0.8FIG.neno(μm)R0 > 0.8)T90 > 0.8)R0 > 0.8)θa range (Note 2)1B1.5891.55.071°, 89°0°, 89°71°, 89°19°1C1.5891.3895.061°, 89°0°, 89°61°, 89° (Note 1)29°1D1.3891.3890.561°, 89°0°, 89°61°, 89° (Note 1)29°(Note 1) - Best theoretical values given.(Note 2) - The θa range includes all angles in the given overlap range.
As the comparative examples of FIGS. 1B-1D show, conventional solutions to remedy this limitation can be severely hampered by the physical characteristics of the optical materials themselves. For example, this constraint in functional θa range from 71-90 degrees could be eased somewhat if a birefringent material having sufficiently large birefringence were to be used. A birefringence larger than 0.35, for example, would alleviate this problem.
However, materials with large birefringence and other desired properties are not easily available, may not be usable for reflective polarizer use, or may not even exist. Light guide plate 12 must have an index of refraction nLGP that is substantially equal to the larger of the extraordinary index ne and ordinary index no. The smaller of the extraordinary index ne and ordinary index no is usually greater than 1.50, which means that the light guide plate must have relatively large index of refraction nLGP, for example, that of polycarbonate, 1.589. However, this is undesirable or unworkable, because the most commonly used light guide plate is made of poly(methyl methacrylate) (PMMA) with index of refraction of around 1.49. Thus, solutions using high levels of birefringence are constrained by properties of the dielectric materials themselves.
Clearly, a good portion of the light incident from light source 14 (FIG. 1A) is not used when the reflective polarizer solution described by Li et al. and shown in FIG. 1 of the '423 disclosure is employed. Again, with this type of conventional solution, while light in the 51-90 degree θa range is available, acceptable polarization separation is provided only for light that is in the 71-90 degree range. For the materials most commonly used for light guide plates, no suitable solution has yet been provided for a reflective polarizer that works over the full θa range of light angles within the light guide plate. Attempts to alleviate this problem using conventional approaches have been thwarted by the limitations of the optical materials themselves. There is, therefore, a need for an illumination solution that provides polarized light over a broader incident angle range than is permissible when using conventional reflective polarizer techniques.